Smog is nasty enough in the atmosphere, but now research suggests that ozone, a key component of smog, stresses out human skin cells.

Cars and factories belch pollutants into the air that combine with the sun's rays to form photochemical smog. Ozone in the lower atmosphere contributes to the smog that's visible to the eye; this is different from ozone high in the atmosphere that helps protect life on Earth from deadly doses of ultraviolet solar radiation.

Researchers at the University of Wisconsin, Madison, exposed human skin cells to the smog-related ozone in the laboratory and found that they turned on cellular machinery that normally responds to stress, suggesting ozone may be toxic to human skin. However, further experiments are required to confirm the findings in people.

Smog breaks down into free radicals when zapped by the sun. These free radicals bounce around inside cells like pinballs, destroying most of the "machinery" they hit. Free radical damage has been implicated in diseases such as cancer, heart disease, Alzheimer's disease and Parkinson's disease.

While smog's damaging effects on our respiratory system have been well studied, little is known about how smog affects our skin, even though urban and suburban residents are repeatedly exposed to ozone on smoggy days.

The lab research involved isolating and exposing normal skin cells to ozone at 0.3 parts per million. Typical ozone levels in big cities can range from 0.2 to 1.2 ppm.

In the lab, ozone exposure boosted the activity of enzymes that convert environmental pollutants and cigarette smoke to more toxic compounds.

The study was published in the June 18 issue of the Journal of Investigative Dermatology.